Sizing a Tankless Water Heater

Sizing a tankless water heater is different from sizing the
tank-type heating appliances. The rule is still the same; you want your
tankless model to provide sufficient hot water at a minimum temperature
of 120 F during the peak time of the day.

Like with the tank-type heaters,
you don't want to buy an oversized or undersized tankless unit, because
it brings unnecessary
issues, rising the operating costs; affecting your budget and
time.

To
determine which tankless water heater model is right for you and what
is the correct size or capacity you should buy, there are
several factors that have to be taken into account.

Things to consider

How much water will be required during the busiest usage
period or peak
time can be determined based on the following factors:

Size of the home and family.

How many bathrooms, tubs, dishwasher and washers are going
to be used? Are you planning any additions in the future?

Do you have kids or teenagers as the hot water usage
is increasing with their needs?

What is the pattern of the usage?

Quantity and type of fixtures.

Here are the consumption rates as an indication of how much
water is
required by the certain application. Use it as a reference when sizing
a
tankless water heater:

For shower or bath you need around 20 gallons.

Showers with energy efficient shower head - 2.5-3
gal/min.

Automatic dishwasher needs around 14 gallons.

Washing machine needs 32 gallons to wash the clothes.

For shaving you need 2 gallons.

How to calculate
the size of a tankless water heater

To calculate what the peak demand for hot water is, list the
number of
devices you are going to use at one time and add up the flow rates. If
the tankless unit is used to supply a shower and two sinks
at the same time and if we assume that the peak demand for
one shower is 2.5 gpm and 0.75 gpm for each sink, your total peak
demand is 4.0 gpm.

Whole house heaters will usually provide 2 gpm to 4
gpm at 75 F temperature rise. Choose the tankless
water heater model closest to the calculated flow rate and temperature
rise needs.

Note: Standby heat loss or energy
wasted when the hot water is not being used, was very important factor
when calculating the tank size; but in a case of tankless
water heaters and lack of storage tanks, standby heat loss can be
completely eliminated. Tankless models also don't have the First hour
rating, as it works instantaneous.

Important
parameters to consider

If
the largest on-demand heater available cannot
meet all
your
residential hot water needs, several tankless (up to
24 with
Noritz multi-system) units can be linked into one system to provide
enough hot
water for every application and any house demand.

As the cold
water passes through the heat exchanger the flow must
have
the right rate to ensure the proper heat transfer. If it is lower
than required by the manufacturer, too much heat from the gas
burner can affect the proper heat exchanger performance, deform it or
if the safety element works properly, shut the unit off.

Many professionals consider the flow
rate
as the most important factor when sizing a tankless water heater. The
flow rate depends upon the temperature of the incoming water. This is
why it is very helpful to determine the climate area where you live
(use a map below if you live in North America) so you can find the
average ground water temperature.

Rinnai manufacturer of
tankless heaters recommends that if the ground water
temperature
is between 65 and 75 F, for three applications, you can use
their model R50LSi (or R94LSi
for four applications) while during the winter
where the incoming water temperature is between 45 and 55 F, you will
use the same model but for two applications instead (R94LSi - three
applications).

The
flow rate can also be determined by calculating the
temperature
rise. The temperature rise is basically how much you need to
heat
the water to reach the desired temperature setting (120 F is the
recommended).

For example, if you set the heater
temperature to 120 F and your area's ground water
temperature is
60 F, the difference is 60 F in temperature
rise. Recommendation
is to choose a residential tankless heater based on the colder
seasons so that you will get the peak demand you need for hot water.

Gas input and heat
power from heating elements tankless units can provide and
energy factor EF, are also
important factors when sizing a
tankless water heater. Residential models for
example, Energy Star
qualified with EF 0.87 or higher have the maximum gas input of 199,900
BTU/hr while many
condensing units are usually above this value.

Ground water
temperature

Since
the incoming water varies throughout the North America and it
fluctuates, there are three climate zones that you have to consider
when sizing a tankless water heater:

Northern zone with the average
ground temperature between
37 F and 51 F.

Central zone with the average
ground temperature between
52 F and 61 F.

Southern zone with the average
ground temperature between

62 F and 77 F.

Conclusion

As
the tankless unit works on-demand and continuously supplying hot water,
the tank size is irrelevant when sizing a tankless water heater.

If the tankless unit is not sized correctly you will have the
output
temperature the same, but the flow will be affected. If
the flow
is higher the temperature is lower. Water should be heated of at least
120 F where the minimum flow rate is usually 2 gallons per minute.

Suggestion is to calculate all the flow rates from the
applications you
will use and take into account the incoming water temperature.
Lower temperature, more BTU you need. If one unit cannot
supply enough hot water, two or more models can be installed by using
the multi-system option.

If thinking between gas tankless heaters and electric, it is
good
to know that gas units can provide hot water at the higher flow rates
than
electric models. Be sure that flow rates will meet the peak demand at
the temperature rise you need.

More about Tankless

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